Transmission systems on optical fiber predominantly use predetermined windows (i.e., bands, channels) of the optical spectrum through which the transmission of the signals along the fibers takes place with minimum attenuation. Signals or communication channels, each with its own precisely defined wavelength as produced by a relevant laser generator, included in one of these windows or bands may be transmitted along an optical fiber with extremely low losses. The simultaneous transmission of various communication channels belonging to a certain band, window or channel on a same fiber is made possible by Wavelength Division Multiplexing (WDM).
Arrayed Waveguide Gratings (AWG) are devices capable of multiplexing a plurality of optical signals at different wavelengths into a single optical fiber, and demultiplexing optical signals at different wavelengths transmitted over a single optical fiber. As a result of this property, they may be used in particular as wavelength division demultiplexers to retrieve individual channels of different wavelengths at the receiving end of an optical communication network.
A schematic diagram of an arrayed waveguide grating (AWG) demultiplexer is shown in FIG. 1. It substantially comprises a first slab waveguide 1 defining a first free propagation region (FPR), a second slab waveguide 2 defining a second free propagation region coupled to the first slab waveguide 1 through an array of optical waveguides 3. The first slab waveguide 1 is coupled to receive multiplexed optical signals of different wavelengths λ1, λ2, λ3, λ4, for example, conveyed through a first optical waveguide 4, and to irradiate them towards first end portions of the waveguides 3. The optical waveguides of the array define optical paths of different lengths. More precisely, each waveguide of the array is shorter by the same fixed length ΔL than the longer adjacent waveguide. Each waveguide is longer by a fixed length ΔL than the shorter adjacent waveguide, except the shortest waveguide.
When the optical signals have crossed the arrayed waveguides, they reach the second end portion thereof from which they are irradiated through the second free propagation region of the second slab waveguide 2. Optical signals of a same wavelength constructively interfere with a maximum intensity at a respective main focal spot located in a position that depends on the wavelength, as shown in FIG. 1.
This device is sensitive to temperature variations or process spread. As schematically shown in FIGS. 2a and 2b, temperature variations may change the optical path difference neff·ΔL between adjacent waveguides, with neff being the effective refractive index of the waveguides. As a consequence, the focal spots of the demultiplexed optical signals may be shifted clockwise or counter-clockwise depending on the variation of the effective refractive index neff upon temperature. To prevent information losses, the number of optical signals at different wavelengths that may be transmitted on a same optical fiber is smaller than the maximum number that in theory could be allowed in absence of temperature variations.
An AWG device is disclosed in the article by Andrew Hang, Cary Gunn, Guo-Liang Li, Yi Liang, Sina Mirsaidi, Adithyaram Narasimha, Thierry Pinguet, “A 10 Gb/s photonic modulator and WDM MUX/DEMUX integrated with electronics in 0.13 μm SOI CMOS”, ISSCC 2006, Session 13, Optical communication, 13.7. This prior AWG has one single array of identical PIN junction phase modulators individually controlled by a dedicated DAC integrated into each arm of the AWG, and may be used to restore the phase relationship of the light due to errors in fabrication of optical waveguides that induce random delays to the optical signal.
In this case, the errors in fabrication may randomly effect any waveguide and, if required, a correction needs to be applied individually on each arm of the AWG. The number of waveguides can easily grow up to 100 or more, and the algorithm and the electronics dedicated to controlling all the DACs become too complex to be practically formed.